The present application corresponds to JP 2000-385659, filed Dec. 19, 2000, and to JP 2000-398231, filed Dec. 27, 2000, the subject matter of which is hereby incorporated herein by reference.
1. Technical Field
The invention concerns a method and apparatus for the biological removal of phosphorus from raw water, and in particular a method and apparatus for the biological removal of phosphorus which has a sludge-reducing solubilizing means, in which phosphorous removal is carried out biologically by means of an anaerobic/aerobic method.
2. Related Art
In an active sludge treatment process where waste water which contains phosphorus is subjected to a biological treatment with active sludge, the amount of phosphorus is reduced along with the degradation of the organic compounds in the raw water. In the metabolism of materials by microorganisms, phosphorus is also included in ATP and nucleic acids (DNA, RNA), and it is an essential element for biomass synthesis. Generally, about 2.3% phosphorus is present in activated sludge, and the removal of the phosphorus with the progress of biomass synthesis can be expected. However, the amount of phosphorus removed in activated sludge treatment, if the average retention time of the sludge is held constant, is roughly proportional to the organic material concentration in the waste water, and so the amount of phosphorus which can be removed is limited.
On the other hand, under certain specified conditions, a large amount of phosphorus other than the phosphorus required for growth and metabolism of the biomass is accumulated in the biomass, and the phosphorus concentration in such activated sludge can reach from 6 to 8%. The phenomenon is called excess uptake or luxury uptake. That is to say, sludge where the environment of the activated sludge has been rendered anaerobic and which has forcibly released phosphorus subsequently exhibits a marked increase in the amount of phosphorus which is taken up.
In an apparatus for the biological removal of phosphorus, use is made of the luxury uptake phenomenon such that return sludge is introduced into an anaerobic tank into which the raw water is introduced and phosphorus is released from the sludge, and the raw water which contains the phosphorus-deficient sludge is subjected to an aerobic treatment and the phosphorus is taken up in excess by the sludge and phosphorus is removed.
In an apparatus for the biological removal of phosphorus of this type, the phosphorus is taken up in the sludge and separated from the water along with the excess sludge, but there is a disadvantage in that a large amount of sludge is formed.
Thus, an apparatus for the biological removal of phosphorus, in which some of the return sludge is left to stand in an anaerobic tank and phosphorus is released and then this is subjected to solid/liquid separation, the solid fraction is solubilized and the amount of sludge is reduced in volume, has been proposed (Japanese Unexamined Laid Open Patent Application H11-188383). Furthermore, an apparatus for the biological removal of phosphorus, in which the return sludge is solubilized and the sludge is reduced in volume and at the same time the phosphorus-containing water which is discharged from the sludge in the anaerobic tank is subjected to a crystallization type phosphorus removal treatment, has been proposed by the present applicant (Japanese Unexamined Laid Open Patent Application H11-57773).
A schematic representation of the phosphorus-removing apparatus of Japanese Unexamined Laid Open Patent Application H11-188383 is shown in FIG. 5, where the raw water 31 is introduced into the anaerobic tank 33 of the biological phosphorous-removal activated sludge treatment process 32 along with return sludge 38 from the settling tank 35 and ozone-oxidized sludge 49 which is discharged from the ozone oxidation tank 48, and BOD elimination and absorption of phosphorus by the phosphorus-removing bacteria are carried out in the anaerobic tank 33 and the aerobic tank 34 which is connected thereto in the process 32. Some of the sludge in the same process is sent to the settling tank 35 and the supernatant water in the settling tank 35 is taken out as treated water 36, and most of the settled sludge 37 which has settled out is recycled to the anaerobic tank 33 as the returned sludge 38.
A part of the sludge is divided off from the aforementioned settled sludge 37 and is poured into the sludge anaerobic tank 40 as the branched or divided sludge 39 and phosphorus is released from the phosphorus-containing sludge on being left to stand here under anaerobic conditions. The sludge in the sludge anaerobic tank 40 is then subjected to solid/liquid separation by means of a membrane separating tank 41, and then the separated sludge 43 obtained by solid/liquid separation is introduced into an ozone-oxidation tank 48 and subjected to oxidative degradation with ozone, and soluble organic material is dissolved out. The ozone-oxidized sludge 49 produced in this ozone-oxidation tank 48 is returned to the anaerobic tank 33 of the biological phosphorous removal activated sludge treatment process 32 and then flows into the aerobic tank 34, and the ozone-oxidized sludge 49 (of which the biodegradability by microorganisms has been improved) is degraded to carbon dioxide gas and water microbiologically.
The activated sludge which has been propagated by elimination of the BOD of the raw water in this way is degraded to carbon dioxide gas and water by circulation through the sludge anaerobic tank 40, the ozone-oxidation tank 48 and the biological phosphorus-removing activated sludge treatment process 32, and so it should be discharged out of the system and the amount of excess sludge is more or less zero.
Moreover, soluble organic matter (BOD) is included in abundance in the ozone-oxidized sludge 49 as a result of the oxidative degradation of the activated sludge with ozone and so, if this is added to the anaerobic tank 33, the phosphorus release from the phosphorus-removing bacteria can be made to occur actively. Furthermore, if the sludge is left to stand anaerobically beforehand in the sludge anaerobic tank 40, the solubilization rate of the sludge is improved in the ozone-oxidation of sludge in the ozone-oxidizing tank 48.
On the other hand, metal compounds which bring about chemical precipitate-forming reactions with phosphorus, such as calcium, magnesium, aluminum or iron compounds for example, are added in the phosphorus chemical-removal process 45 to the solid/liquid separated water 42 from the membrane separating tank 41 and the phosphorus is precipitated and separated and recovered as a calcium phosphate compound, such as hydroxyapatite, magnesium ammonium phosphate (MAP), aluminum phosphate or iron phosphate. In the case of FIG. 5, calcium hydroxide 44 is used for the metal compound and the hydroxyapatite 47 is produced.
The biological phosphorus-removing apparatus of the aforementioned Japanese Unexamined Laid Open Patent Application No. H11-57773 is an apparatus for the biological removal of phosphorus by means of an anaerobic/aerobic method, in which there is provided a sludge solubilizing process to which some return sludge is introduced, a means of returning sludge which has been solubilized in said sludge solubilizing device to the anaerobic tank, and a phosphorus-removal reaction column into which the liquid from the anaerobic tank is introduced.
In that apparatus for the biological removal of phosphorus, the volume of sludge can be reduced by solubilizing at least some of the return sludge and subjecting it to biological treatment once again. Furthermore, by removing and recovering the phosphorus which is included in the solubilized sludge obtained by solubilizing the returned sludge as MAP (magnesium ammonium phosphate) by way of the anaerobic tank and the phosphorus-removing column (MAP reaction column), it is possible to remove the phosphorus from the system.
A schematic drawing which shows the apparatus for the biological removal of phosphorus disclosed in the aforementioned Japanese Unexamined Laid Open Patent Application H11-57773 is shown in FIG. 6, and this comprises an anaerobic tank 1, a MAP reaction column 2, a denitrification tank 3, a nitrification tank (aerobic tank) 4, a settling tank 5 and a solubilizing tank 6. The waste water is introduced into the anaerobic tank 1 together with solubilized sludge from the solubilizing tank 6. In the anaerobic tank 1, under anaerobic conditions, the phosphorus in the solubilized sludge is released to the liquid side (this phosphorus is almost all in the form of orthophosphoric acid which is convenient for MAP formation). This anaerobic treatment liquid is introduced into the MAP reaction column 2 and generally has a phosphorus concentration of from 15 to 25 mg/l and so the MAP formation reaction proceeds smoothly.
In the MAP reaction column 2, an alkali such as NaOH is introduced in such a way as to set the preferred pH conditions of from pH 8 to 10, and most desirably of from pH 8 to 9, for MAP, along with, in those cases where there is insufficient magnesium to precipitate MAP, the addition of a magnesium compound such as MgCl2 or Mg(OH)2, for example, (this should contain a magnesium compound and may be sea water), and the phosphorus and ammonia in the liquid react with the magnesium to form MAP which forms a precipitate, and the phosphorus in the liquid is removed in this way. In particular, the phosphorus in the solubilized sludge is in the form of orthophosphoric acid which is convenient for the formation of MAP as a result of the biological treatment, and the MAP formation reaction efficiency in the MAP reaction column 2 is high and so the removal of phosphorus is carried out efficiently.
The liquid which flows out of the MAP reaction column 2 is generally a liquid which has a phosphorus concentration of some 10 mg/l and this discharged liquid is then introduced into the denitrification tank 3. In the denitrification tank 3, the BOD in the waste-water is utilized and the NO3 and NO2 in the nitrification cycle liquid provide for nitrogen removal.
The liquid from the nitrogen removal treatment is introduced into the nitrification (aerobic) tank 4 and, by aeration, the ammonia in the liquid is oxidized to NO3 and NO2. Furthermore, under aerobic conditions the phosphorus is taken up by the activated sludge and the phosphorus concentration in the liquid is reduced.
Some of the nitrification treatment liquid is returned to the denitrification tank 3 to provide NO3 and NO2 and the rest is supplied to the settling tank 5 and subjected to solid/liquid separation.
The separated liquid from the settling tank 5 is discharged from the system as treated water. This treated water is water of good quality from which the phosphorus and nitrogen have been removed by the formation of MAP in the MAP reaction column 2 and, moreover, nitrogen has been removed in the denitrification tank 3 and phosphorus has been removed in the aerobic tank 4.
On the other hand, at least some of the separated sludge in the settling tank 5, which has taken up phosphorus in the form of orthophosphate as a result of the biological treatment, is introduced into the solubilizing tank 6 and subjected to a solubilizing treatment by blowing in ozone gas. That is to say, the sludge is oxidatively degraded to BOD components and solubilized with ozone.
As well as pure ozone, ozone-containing air or ozonized air, for example, can be used for the ozone treatment gas in the solubilizing tank 6.
As a result of various studies, it has been recognized that the rate of release of phosphorus is slow with the apparatus for the biological removal of phosphorus shown in FIG. 5 because there is virtually no substrate in the sludge anaerobic tank. Furthermore, because the sludge in the phosphorus-releasing tank is concentrated and the whole amount is solubilized, phosphorus removal and the reduction in sludge volume are even and the extent of the two actions are not interrelated and are difficult to set. Furthermore, in the apparatus for the biological removal of phosphorus shown in FIG. 6, it has been confirmed that the water which is introduced into the MAP reaction column contains a large amount of sludge so that the MAP production reaction is impeded.
The present invention is intended to provide an apparatus for the biological removal of phosphorus with which the volume of sludge is reduced and the phosphorus removal efficiency is increased.
According to one embodiment of the present invention, a biological method of removing phosphorus from raw water, comprises releasing phosphorus from activated sludge in an anaerobic tank in a phosphorus-releasing step, absorbing the phosphorus with activated sludge in a mixed liquor of the raw water and the activated sludge in an aerobic tank in a phosphorus-absorbing step, separating sludge from the mixed liquor in a first solid/liquid separation step, supplying the sludge which has been concentrated and separated from the mixed liquor in the first solid/liquid separation step to the anaerobic tank, subjecting the sludge from the phosphorus-releasing step to a second solid/liquid separation step, solubilizing sludge which has been concentrated and separated in the second solid/liquid separation step in a solubilizing step, supplying solubilized sludge of the solubilizing step to the anaerobic tank, and removing phosphorus from liquid which has been separated in the second solid/liquid separation step.
According to the aforementioned embodiment some of the solubilized sludge from the solubilization process is supplied to the phosphorus-releasing process, thereby increasing the substrate concentration in said phosphorus-releasing process. Consequently, the phosphorus release rate in the phosphorus-releasing process is increased and the phosphorus removal can be carried out with better efficiency.
In a further aspect of the aforementioned embodiment, there is a by-pass process in which some of the separated sludge which has been concentrated and separated in the aforementioned second solid/liquid separation process is supplied to the aforementioned phosphorus-releasing process, bypassing the aforementioned solubilization process.
According to this further aspect all of the separated sludge which is concentrated and separated in the second solid/liquid separation process is not supplied to the solubilization process and some of the aforementioned separated sludge is supplied to the aforementioned phosphorus-releasing process, bypassing the aforementioned solubilization process, and so the amount of phosphorus removed and the extent of the reduction in the volume of sludge can be each be set independently and the operation of the apparatus for the execution of the invention is simplified.
In a further aspect of the aforementioned embodiment, the aforementioned solubilized liquid supply process includes a fermentation treatment process in which the solubilized liquid of the aforementioned solubilization process is subjected to a fermentation treatment, and the fermented liquid of said fermentation treatment process is supplied to the aforementioned phosphorus-releasing process.
According to this aspect, it is desirable that the anaerobicity in the anaerobic tank be enforced and that the release of phosphorus be accelerated. For this reason, the sludge which has been solubilized in the solubilization process is preferably introduced into the phosphorus-releasing process via a fermentation treatment process.
An apparatus for the biological removal of phosphorus from raw water according to one embodiment of the present invention comprises an anaerobic tank into which the raw water and activated sludge are introduced to form a mixed liquor, an aerobic tank in which the mixed liquor from the anaerobic tank is aerated, a settling tank in which the mixed liquor flowing out of said aerobic tank is subjected to solid/liquid separation, a first pipe connecting the settling tank to the anaerobic tank whereby some of the sludge separated from the mixed liquor in the settling tank is returned as the activated sludge to the anaerobic tank, a phosphorus-releasing tank in which a remaining portion of the separated sludge from the settling tank is maintained anaerobically and phosphorus is released from the sludge, a solid/liquid separating device in which sludge admixed liquid from the phosphorus-releasing tank is subjected to solid/liquid separation, a solubilizing device in which sludge separated in said solid/liquid separating device is solubilized, a solubilized sludge returning device with which sludge which has been solubilized in said solubilizing device is returned to the anaerobic tank, a second pipe for supplying some of the sludge which has been solubilized in the solubilizing device to the phosphorus-releasing tank, and a phosphorus-removing device in which phosphorus is removed from water which has been separated in the solid/liquid separating device.
In the aforementioned apparatus for the biological removal of phosphorus, some of the solubilized sludge from the solubilizing tank is supplied to the phosphorus-releasing tank, thereby raising the substrate concentration in said phosphorus-releasing tank. Consequently, the phosphorus-releasing rate in the phosphorus-releasing tank is increased and it is possible to carry out the removal of phosphorus more efficiently.
An apparatus for the biological removal of phosphorus according to another embodiment of the present invention includes an anaerobic tank into which raw water and returned sludge are introduced to form a mixed liquor, an anoxic tank in which bound oxygen is applied to the mixed liquor from said anaerobic tank, an aerobic tank in which the mixed liquor from said oxygen-free tank is aerated, a mixed liquor returning means which returns some of the mixed liquor from said aerobic tank to the aforementioned oxygen-free tank, a first solid/liquid separating means in which the liquid flowing out of said aerobic tank is subjected to solid/liquid separation, a means whereby some of the solid/liquid separated sludge from said first solid/liquid separating means is returned as the aforementioned returned sludge to the aforementioned anaerobic tank, a phosphorus-releasing tank in which the remaining solid/liquid separated sludge from the aforementioned first solid/liquid separating means is maintained anaerobically and the phosphorus is released, a second solid/liquid separating means in which the sludge admixed liquid from said phosphorus-releasing tank is subjected to solid/liquid separation, a solubilizing means in which the sludge separated in said second solid/liquid separating means is solubilized, a solubilized sludge returning means with which sludge which has been solubilized in said solubilizing means is returned to the aforementioned anaerobic tank, a means of supplying some of the sludge which has been solubilized in the aforementioned solubilization means to said phosphorus-releasing tank, and a phosphorus-removing means with which phosphorus is removed from separated water which has been separated in the aforementioned second solid/liquid separating means.
In the immediately aforementioned apparatus for the biological removal of phosphorus, some of the solubilized sludge from the solubilizing tank is supplied to the phosphorus-releasing tank, thereby increasing the substrate concentration of said phosphorus-releasing tank. Consequently, the phosphorus-releasing rate in the phosphorus-releasing tank is increased and the removal of phosphorus can be carried out more efficiently. Furthermore, an anoxic tank is established and so it is possible to carry out the removal of nitrogen at the same time as the removal of phosphorus.
In either of the aforementioned apparatus for the biological removal of phosphorus, the aforementioned solubilized sludge returning means may be furnished with a fermentation tank, into which the sludge which has been solubilized in the aforementioned solubilizing means is introduced and subjected to a fermentation treatment, and there may be provided a means whereby the sludge which has been subjected to a fermentation treatment in said fermentation tank is returned to the aforementioned anaerobic tank.
In this embodiment, it is desirable that the anaerobicity in the anaerobic tank is enforced and phosphorus release is promoted. Consequently, the sludge which has been solubilized in the solubilizing means is preferably introduced into the anaerobic tank after a fermentation treatment.
In the immediately aforementioned apparatus for the biological removal of phosphorus, there is provided a means of supplying some of the sludge which has been subjected to a fermentation treatment in the aforementioned fermentation tank to the aforementioned phosphorus-releasing tank.
It is desirable that some of the sludge which has been subjected to the fermentation treatment should be supplied to the phosphorus-releasing tank in order to enforce the anaerobicity of the anaerobic tank and promote the release of phosphorus.
In any of the aforementioned apparati for the biological removal of phosphorus, there may be provided a means whereby some of the phosphorus-releasing sludge which has been separated in the aforementioned second solid/liquid separating means is supplied as it is to the aforementioned anaerobic tank.
With the immediately aforementioned apparatus for the biological removal of phosphorus, not all of the separated sludge which has been concentrated and separated in the second solid/liquid separating means is supplied to the solubilizing tank and some of the aforementioned separated sludge is supplied to the aforementioned anaerobic tank, bypassing the aforementioned solubilizing tank, and so it is possible to set the amount of phosphorus removed and the reduction in volume of the sludge independently, and the operation of the apparatus is simplified. That is to say, some of the phosphorus-releasing sludge which has been separated in the second solid/liquid separating means is supplied as it is to the anaerobic tank without passing through the solubilizing means, thereby providing an advantage in that, even if there are changes in the phosphorous concentration and BOD concentration of the raw water such as an industrial effluent, adjustments can be made to achieve the required phosphorus removal and to carry out a reduction in the excess sludge volume.
An apparatus for the biological removal of phosphorus according to another embodiment of the present invention includes an anaerobic tank into which raw water and return sludge are introduced, an aerobic tank in which the mixed liquor from said anaerobic tank is aerated, a first solid/liquid separating means in which the liquid flowing out of said aerobic tank is subjected to solid/liquid separation, a means whereby at least some of the solid/liquid separated sludge from said first solid/liquid separating means is returned as the aforementioned return sludge to the aforementioned anaerobic tank, a second solid/liquid separating means in which some of the mixed liquor discharged from the aforementioned anaerobic tank is subjected to solid/liquid separation, a solubilizing means in which at least some of the sludge separated in said second solid/liquid separating means is solubilized, a solubilized sludge returning means with which sludge which has been solubilized in said solubilizing means is returned to the aforementioned anaerobic tank, and a phosphorus-removing means with which phosphorus is removed from the separated water which has been separated in the aforementioned second solid/liquid separating means.
In the aforementioned apparatus for the biological removal of phosphorus, return sludge is introduced into the anaerobic tank into which the raw water is introduced and phosphorus is released from said returned sludge. The anaerobic treatment water which contains this released phosphorus is subjected to a solid/liquid separation treatment, and just the separated water is supplied to the phosphorus-removing means, and so the phosphorus removal efficiency of the phosphorus-removing means is increased. Furthermore, the solid fraction separated in the second solid/liquid separating means is subjected to a solubilizing treatment and the volume of sludge is reduced.
An apparatus for the biological removal of phosphorus according to another embodiment of the present invention includes an anaerobic tank into which raw water and return sludge are introduced, an oxygen-free tank in which bound oxygen is administered to the mixed liquor from said anaerobic tank, an aerobic tank in which the mixed liquor from said oxygen-free tank is aerated, a first solid/liquid separating means in which the liquid flowing out of said aerobic tank is subjected to solid/liquid separation, a means whereby at least some of the solid/liquid separated sludge from said first solid/liquid separating means is returned as the aforementioned return sludge to the aforementioned anaerobic tank, a second solid/liquid separating means in which some of the mixed liquor discharged from the aforementioned anaerobic tank is subjected to solid/liquid separation, a solubilizing means in which at least some of the sludge separated in said second solid/liquid separating means is solubilized, a solubilized sludge returning means with which sludge which has been solubilized in said solubilizing means is returned to the aforementioned anaerobic tank, and a phosphorus-removing means with which phosphorus is removed from the separated water which has been separated in the aforementioned second solid/liquid separating means.
With the aforementioned apparatus for the biological removal of phosphorus, it is possible to remove phosphorus and reduce the sludge volume, and nitrogen removal is also possible.
In either of the two immediately mentioned apparati for the biological removal of phosphorus, there may be provided a means whereby the water, from which the phosphorus has been removed in the aforementioned phosphorus-removing means, is subjected to an aerobic treatment or whereby said water is supplied to the aforementioned oxygen-free tank or the aforementioned aerobic tank.
With the aforementioned apparatus for the biological removal of phosphorus, the organic material which is included in the water from which the phosphorus has been removed in the aforementioned phosphorus-removing means is degraded.
In any of the above-mentioned apparati for the biological removal of phosphorus, the aforementioned solubilized sludge returning means may be means which supplies some of the sludge which has been separated by the aforementioned second solid/liquid separating means to the aforementioned solubilizing means, and there is provided a means which supplies the remainder of the sludge which has been separated by said second solid/liquid separating means to the aforementioned anaerobic tank or the aforementioned aerobic tank.
With the aforementioned apparatus for the biological removal of phosphorus, not all of the separated sludge which is concentrated and separated in the second solid/liquid separating means is supplied to the solubilizing tank, and some of the aforementioned separated sludge is supplied to the aforementioned anaerobic tank or the aforementioned aerobic tank, by-passing the aforementioned solubilizing tank, and so it is possible to set the amount of phosphorus removal and the reduction in sludge volume independently, and operation of the apparatus is facilitated.
In any of the four immediately mentioned apparati for the biological removal of phosphorus, the aforementioned solubilized sludge supply means may be furnished with a fermentation tank, into which at least some of the sludge which has been solubilized in the aforementioned solubilizing means is introduced and subjected to a fermentation treatment, and the sludge which has been subjected to the fermentation treatment in said fermentation tank is returned to the aforementioned anaerobic tank.
In this embodiment, it is desirable to enforce the anaerobicity in the anaerobic tank and promote the release of phosphorus. Consequently, it is desirable to introduce the sludge which has been solubilized in the solubilizing means into the anaerobic tank after it has been subjected to a fermentation treatment.
In the immediately aforementioned apparatus for the biological removal of phosphorus, the aforementioned solubilized sludge returning means may supply only some of the sludge which has been solubilized in the aforementioned solubilizing means to the aforementioned fermentation tank, and there may be provided a means by which the remainder sludge which has been solubilized in said solubilizing means is supplied to the aforementioned second solid/liquid separating means.
With the aforementioned apparatus for the biological removal of phosphorus, the phosphorus dissolved out from the sludge by solubilization can be transferred into the separated liquid in the second solid/liquid separating means, and so the phosphorus removal efficiency is increased.
In any of the aforementioned apparati for the biological removal of phosphorus, there may be provided a means which supplies some of the sludge which has been subjected to a fermentation treatment in the aforementioned fermentation tank to the aforementioned second solid/liquid separating means.
With the aforementioned apparatus for the biological removal of phosphorus, the phosphorus dissolved out from the sludge by solubilization can be transferred via the aforementioned fermentation tank to the separated liquid in the aforementioned second solid/liquid separating means, in the same way as in the previously described apparatus for the biological removal of phosphorus, and so the phosphorus removal efficiency is increased.
In any of the three immediately aforementioned apparatus for the biological removal of phosphorus, there may be provided a means whereby some of the raw water is supplied to the aforementioned fermentation tank.
A substrate such as organic material brings about the release of phosphorus in the anaerobic tank, and organic acids are especially effective as substrates. With the aforementioned apparatus for the biological removal of phosphorus, some of the raw water is supplied to the aforementioned fermentation tank, the fermentation treatment is carried out and organic acids are produced, and these are introduced into the anaerobic tank, and so the phosphorous release rate in the anaerobic tank is increased and it is possible to carry out phosphorus removal more efficiently.
The apparatus of this invention can be used to treat various types of phosphorus-containing water, such as town sewage, industrial effluent and river water for example.